Project Summary/Abstract In order to harness the potential of cell therapies, more needs to be understood about cells post transplantation. Our goal is to experimentally validate magnetoendosymbionts (MEs) as a living MRI contrast agent to provide this insight into stem cell engraftment and viability in cardiac and neural injury model by demonstrating live cell specificity (LCS), in vivo. Bioluminescence and commercial MRI contrast agents will be used as controls for validation and to demonstrate competitive advantages. Reporter gene approaches have LCS but suffer from other complications such as need for genetic engineering that complicates regulatory developments. Existing MRI contrast agents provide full anatomic access, but lack LCS due to uptake by macrophages and nonspecific signal. Our preliminary results suggest that MEs can be used to successfully label cardiomyoctyes (iCMs) and human neural progenitor cells (hNPCs), without perturbing cell function. ME- labeled iCMs were successfully engrafted and visualized for 2 weeks in vivo (the overall goal of Phase 1). Moreover, preliminary results suggest MEs provide LCS whereas the passive MRI agent did not. In Phase 2, we propose to extend from this positive progress and fully demonstrate the value of ME- based cell tracking in a second model (hNPCs) and firmly experimentally define the LCS competitive advantage in both models. Development of imaging reagents that can effectively and specifically label cells in vivo with minimal toxicity addresses critical barrier for the cell therapies. Such a tool will lower the risk of capital investments forR&D and clinical trials, by providing the information on bio-distribution and viability needed to optimize cell therapies.